1: Go to the folder where you have installed Catacombs.
2: Delete the folder called 'Language'
3: Open the file 'update.dat' using any text editor such as Notepad or Word.
4: In the file are two URLs which will look something like - patch.camelot-europe.com:1280/updates/german/catacombs/mpatch.dat
5: Change the word 'german' (or 'french' depending on where you bought it) to 'english' in both URLs then save the file.
6: Connect to the game, you'll download a small patch then your client will be in English.